1. Field of the Invention
The invention relates to television tuning systems and methods, and in particular to television tuning systems and methods employing use of ghost cancellation and antenna directionality control to produce a television signal from which images which are substantially ghost-free can be obtained.
2. Description of Related Art
Despite the fact that cable television installations are on the rise and more and more televisions are receiving television signals from cable, there is still a significant number of televisions, including an increasing number of portable televisions, which receive television signals "from the air" via antennas. The images which appear on the display of a television which receives television signals "from the air" are often bothered by echoes, commonly referred to as "ghosts". Such ghosts occur as a result of a transmitted original television signal being interfered with and altered by other signals, e.g., reflected signals resulting from reflections of the transmitted original television signal by tall buildings and/or mountains, as the transmitted original television signal travels "through the air". Such reflected signals have different delay times and amplitudes than the transmitted original television signal, causing the images which are displayed on a television display from a received television signal (which has been interfered with and altered) to have superimposed copies thereof, i.e., ghosts. These ghosts are classified as pre-ghosts and post-ghosts depending on whether the signals of the received television signal which produce them are received before or after the main signal of the received television signal which corresponds substantially to the transmitted original television signal.
Until recently, the only approach available for trying to eliminate ghosts from a television which receives television signals "from the air" was to change the reception characteristics of the television's antenna, and thereby the reception quality of the television signals received, by either physically rotating the antenna (if it is a rotatable antenna) or adjusting the electronic phase of the antenna (if it is a phase adjustable antenna). (The position or phase of an antenna is hereafter referred to as the "directionality" of the antenna, and rotating the position or adjusting the phase of an antenna is hereinafter referred to as "changing the directionality" of the antenna.) Under this antenna directionality change approach (hereinafter referred to as the "adc approach"), a viewer changes the directionality of an antenna until reception of ghosts is minimized and/or reception of the main signal is maximized. In general, this is a trial and error procedure which often only minimizes, rather than solves, the ghosting problem. In addition, each time the directionality of the antenna is changed, it may be necessary for a tuner which is used in the process of obtaining images, from a television signal received by the television for a selected channel, on a television display to change its tuning frequency. (It is noted that each channel has a different television signal associated with it.) Still further, since the characteristics of a transmission path varies from channel to channel, changing the directionality of the antenna is usually necessary whenever a different channel is selected by a the viewer.
Recently, a second approach for trying to eliminate ghosts was developed. This second approach (hereinafter referred to as the "baseband approach") involves performing a ghost cancellation operation on a television signal after it has been received and demodulated, i.e., at baseband. A good overview to this approach can be found in W. Ciciora et al., "A Tutorial On Ghost Cancelling in Television Systems", IEEE Trans. on Consumer Elec., Vol. CE-25, No. 1, pp 9-44 (Feb. 1979) which is incorporated herein by reference.
In the baseband approach, a transmission channel is characterized (at least those parameters which contribute to signal interference and alteration) by transmitting a ghost cancellation reference signal (hereinafter referred to as a "GCR signal") having known characteristics over the transmission channel as part of a television signal. As a result, both the television signal and the GCR signal are subjected to the characteristics of the transmission channel. After reception and demodulation of the television signal, the received GCR signal (as altered by the channel characteristics) is compared to the original GCR signal, and filter coefficients reflecting the differences between the original and received GCR signals are calculated. These coefficients (hereinafter referred to as "ghost cancellation coefficients") are provided to digital IIR and/or FIR filters in order to model an inverse of the channel characteristics. These digital filters process the complete television signal using the inverse channel characteristics and thereby significantly reduce the effects of ghosts on the images obtained from the received television signal. Methods and apparatus working according to the baseband approach are described in U.S. Pat. Nos. 5,047,859, 5,111,298, 5,121,211, and 5,172,232, which are all incorporated herein by reference.
A problem with the baseband approach is its that its ability to perform ghost cancellation, and the quantity of such ghost cancellation when it can be performed, is limited by the reception quality of a television signal received by a television. Accordingly, if the television signal which is received by the television and supplied to a baseband ghost cancellation unit is of poor reception quality, the baseband ghost cancellation unit may either not be able to perform ghost cancellation on that signal, or if it can perform ghost cancellation, not be able to perform sufficient ghost cancellation to produce a television signal from which substantially ghost-free images can be obtained (that type of television signal is hereinafter referred to as a "ghost-free television signal").
In addition (and somewhat in accord with the adc approach), each time a viewer changes channels, new ghost cancellation coefficients must be calculated, since, as mentioned above, the characteristics of a transmission path varies from channel to channel. This provides a delay in ghost cancelling occurring each time a different channel is selected by a viewer.